The present invention relates to apparatus and methods for forming single wall or dual wall plastic pipe and corrugations and other shapes therein, particularly apparatus and methods for forming transverse corrugations and end shapes on a tubular polymer melt parison formed by an extrusion die. Large diameter single wall or dual wall corrugated plastic pipe so formed is generally used in drainage and sanitary sewer applications. Mold management systems and methods for managing molds in the extrusion process are disclosed.
Typical apparatus for producing corrugated thermoplastic pipe utilizes mold halves that are transferred laterally toward an extrusion die, forming two lines of adjacent and mating mold halves. These two lines of adjacent mating mold halves are transported in a circulating path connected or unconnected to form a continuous moving mold tunnel. At the downstream end of the mold tunnel, mating mold halves are separated and transferred laterally away from the mold tunnel a distance sufficiently large to clear an axial return path upstream (toward the extrusion die) where the pair of molds are laterally transferred again toward each other and the extrusion die where the molds are mated to continually repeat the cycle. In the production of a pipe section, end shapes such a bell or spigot at opposite ends define a predetermined pipe section length.
In the prior art, apparatus are described for producing corrugated thermoplastic pipe by utilizing a plurality of separated mold halves that are transferred laterally toward the extrusion die until the pair of mold halves mate so that the axis of the pair of mold halves is collinear with the axis of the extrusion die. The mated pair of mold halves is subsequently transferred axially downstream from the extrusion die forming two lines of adjacent mating mold halves. These two lines of adjacent mating mold halves are circulated in a path connected or unconnected to form a continuous moving mold tunnel. The speed of the mold halves in the mold tunnel matches the extrusion rate of the tubular polymer melt parison. The pairs of mold halves are laterally transferred from the downstream end of the mold tunnel a distance sufficient to clear a return path to the extrusion die end of the mold tunnel where the pair of molds are laterally transferred again toward each other and the extrusion die to continually repeat the cycle. Horst Rahn in U.S. Pat. No. 4,787,598 describes a corrugating apparatus that laterally transports the mold halves to form a return line by utilizing conveyors to drag the mold bases horizontally around on the bottoms of mold halves. Horst Rahn achieves the circulation of mold halves utilizing a conveyance means to place the mold halves in position to be circulated through the mold tunnel and returned from the end of the mold tunnel to the extrusion die head. This invention has the disadvantage of having to pause the operation of the corrugating apparatus to allow for a time-consuming sequential introduction and removal of mold bases by external means from the corrugating apparatus.
Ralph-Peter Hegler in U.S. Pat. No. 5,693,347 discloses an apparatus that achieves the circulation of mold halves utilizing a pick and place mechanism to place the mold halves in position to be circulated through the mold tunnel and returned from the end of the mold tunnel to the extrusion head. Hegler's apparatus has the capability of sequentially removing a pair of mold halves from the circulating cycle of the corrugating apparatus and placing it on a parking station and introducing another pair of mold halves by transporting them from another parking station. Hegler's mechanism has an advantage over Horst Rahn's by reducing the friction caused by dragging molds by lifting the mold halves off the base during the lateral transfer and the return of the mold halves. However, Hegler has the disadvantage of having a single pick and place mechanism that sequentially transports a mold half to a parking station inline with other parking stations thereby blocking the path of all other mold halves in parking stations. This results in a further disadvantage associated with the corrugating apparatus having to pause to allow the mold half placed in the initial parking station to be removed by an external means. (1) The axial speed of the pick and place mechanism, (2) how far the pick and place mechanism must travel beyond the mold half tunnel to reach the desired parking stations, and (3) the time needed to remove the mold half from the corrugating apparatus by an external means limit the number of accessible mold halves.
In the prior art mentioned above, the movement of the mold halves through the mold tunnel is accomplished by driving a mating pair of mold halves at the beginning of the mold tunnel that push the group of adjacent mating pairs of mold halves downstream through the mold tunnel. This method of moving the mold halves through the mold tunnel has the inherent disadvantage in that the corrugating apparatus can not remove all mold halves between production runs for maintenance of the mold halves, die or corrugator apparatus. The mold halves downstream from the driven mold halves in the mold tunnel are typically removed from the corrugating apparatus by a difficult and time-consuming manual procedure that can be a hazard due to the very large size and weight of the mold halves.
Presently, manufacturers of large diameter corrugated single and dual wall plastic pipe are burdened with the problem of long setup times associated with changing over the production setup of a corrugating apparatus from one pipe size to another. It is common for this process to require a minimum of 4 to 8 hours for two or three persons with the proper equipment to change from producing pipe of one size and shape to another. The industry associated with manufacturing dual wall corrugated plastic pipe now decides how long to make a production run based on the anticipated down time required to make the change over to another product and the cost of carrying additional inventory.